The present invention relates generally to a method for manufacturing a mold for a hot-runner injection molding machine, and more particularly, to a method for manufacturing a mold for a hot-runner injection molding machine for injection molding of metal having a higher melting point and a higher thermal conductivity than resin.
Due to its capability of molding products without runners and sprues, the runnerless injection molding method has a remarkable advantage over the cold-runner system injection molding. Such a runnerless injection molding is suited to injection molding of resin having a relatively low melting point and a low thermal conductivity. The runnerless injection molding method is thus in wide use for the resin injection molding.
FIG. 9 is a sectional view of a mold for a hot-runner injection molding machine making use of induction heating.
The mold comprises a fixed mold plate 3xe2x80x2 having thereon mounted a nozzle 1xe2x80x2 and a manifold 2xe2x80x2, and a movable mold plate 4xe2x80x2 having a cavity 4axe2x80x2 shaped correspondingly to the shape of products. The cavity 4axe2x80x2 is formed in a heat-resistant metallic core 6xe2x80x2 attached to the movable mold plate 4xe2x80x2, whilst a metallic core 5xe2x80x2 corresponding to the metallic core 6xe2x80x2 is attached to the fixed mold plate 3xe2x80x2.
A back plate 8xe2x80x2 is mounted behind (upper side in FIG. 9) the fixed mold plate 3xe2x80x2, with the manifold 2xe2x80x2 being arranged in a space 7xe2x80x2 that is defined between the back plate 8xe2x80x2 and the fixed mold plate 3xe2x80x2. The fixed mold plate 3xe2x80x2 and the metallic core 5xe2x80x2 are formed with a nozzle fitting hole 3axe2x80x2 extending from the space 7xe2x80x2 toward the cavity 4axe2x80x2 of the movable mold 4xe2x80x2. The nozzle 1xe2x80x2 is inserted from the space 7xe2x80x2 into the nozzle fitting hole 3axe2x80x2.
A coil (not shown) is wound around the nozzle 1xe2x80x2 so that the material within the nozzle 1xe2x80x2 is heated by induction heating by the coil.
By the way, the above mentioned mold of the hot-runner injection molding machine is exclusively used for resin injection molding, although it would theoretically be applicable also to injection molding of metals such as magnesium alloy, aluminum alloy and zinc alloy.
For example, Japan Patent Laid-open Publication No. Hei 9-85416 proposes a hot-runner mold capable of injection molding of metal materials such as magnesium alloy, aluminum alloy and zinc alloy.
In characteristics, however, the above metals have a melting point of 400xc2x0 C. to 700xc2x0 C. which is fairly higher than that of resin, and have a fairly higher thermal conductivity than that of resin.
Accordingly, direct application to molten metal injection molding of the existing mold of the hot-runner injection-molding machine for use with resin will pose problems, which follow.
(1) Due to an extremely large difference in temperature between the high-temperature molten metal (material) and the mold, simultaneously with the injection molding the heat of the material is rapidly absorbed by the mold in contact with the nozzle and by the product solidified in the cavity. Thus, for solidifying, the temperature of the material in the gate cut portion drops to the vicinity of the mold temperature, which is fairly lower than the melting point of that material. For this reason, in order to melt the material in the gate cut portion to open the gate cut portion for the next injection, the material in the nozzle runner needs to be heated up to several hundred degrees or above. This means that much time is spent on opening the gate, and it may disturb the high-cycle operation.
(2) Meanwhile, upon the mold opening after the injection molding, the temperature of the material within the gate cut portion and the nozzle in proximity thereto needs to be dropped to a sufficiently low level for solidifying. Otherwise, the molten material may leak out of the nozzle tip or molten material lying behind the gate cut portion may be ejected from the nozzle tip.
(3) In the invention as recited in Japan Patent Laid-open Publication No. Hei 9-85416 described above, the gate is formed with a 0.1 to 0.5 mm diameter circular hole or slit so that the flow resistance of the molten metal passing through the gate becomes higher than the residual pressure of the molten metal material existing in the flow passage, to thereby prevent the molten metal from leaking out of the gate.
Due to the constant exposure of the molten material from the gate, however, any leakage of the molten material is apt to occur upon the mold opening.
From the above reasons, in spite of its higher material yield and productivity, the metal injection molding by the hot-runner injection molding machine was extremely difficult to practice in actuality.
The present invention has been conceived in order to solve the above problems and to provide a method for manufacturing the hot-runner injection molding applicable to metals as well. It is therefore an object of the present invention to provide a method of manufacturing a mold for a hot-runner injection molding machine suitable for the injection molding of molten metal such as molten magnesium alloy and capable of injecting metal by securely blocking the gate cut portion with solidified metal upon the mold opening and by rapidly opening the gate cut portion upon the next injection molding.
The above object is attained by providing a mold having a gate cut portion whose position has been selected in an appropriate manner.
According to the present invention, there is provided a mold for a hot-runner injection molding machine, the mold being provided with a movable mold plate having a cavity and with a fixed mold plate having a nozzle for injecting molten metal into the cavity and having heating means for heating metal existing in the nozzle, the mold comprising temperature measurement means arranged in the vicinity of a gate cut portion where gate cutting is performed, for measuring the temperature of metal in the gate cut portion; heating control means for providing a control of heating of the nozzle effected by the heating means, on the basis of the result of measurement by the temperature measurement means; the gate cut portion formed on the nozzle at a predetermined position thereof; and heat insulation means arranged on the nozzle so as to cover at least an area where the gate cut portion is formed.
The temperature measurement means detects the temperature of the gate cut portion and sends the result of detection to the heating controller. The heating controller compares for example a preset temperature with the detected temperature, and if it is judged that the temperature of the gate cut portion is lower than the preset temperature, outputs a command signal to the heating means so as to heat the nozzle. This allows the temperature of metal in the gate cut portion to be kept at a certain level or more, making it possible to rapidly melt the metal in the gate cut portion by a slight heating upon the next injection molding, rendering the metal injectable.
The heat insulation means reduces the quantity of heat migrating from the gate cut portion to the mold. The reason for the provision of such heat insulation means is as follows.
If the gate cut portion and the gate portion near the gate cut portion are in contact with the mold, then a greater quantity of heat will be radiated from the metal in the gate cut portion to the mold. For this reason, even though the heating means applies heat to the nozzle to keep the temperature of the metal in the gate cut portion at a certain level or more, a lot of quantity of heat will be migrated toward the mold, making it difficult to keep the temperature at a certain level. Additional thermal energies will be needed for heating. Particularly, even in the cases where a remarkably increased difference exists between the temperature of metal within the nozzle runner and the temperature of metal in the gate cut portion, with the temperature of the metal in the gate cut portion being lower than the melting point of that metal, the temperature of metal in the runner may exceed the melting point under the operating of the heating means, with the result that high-temperature molten metal in the runner may fuse the metal in the gate cut portion and leak out from the nozzle tip or may be ejected therefrom. Thus, in order to obviate the above deficiencies by reducing the variance of temperature between the interior of the nozzle, especially, the gate cut portion and the runner, the heat insulating means is disposed around the gate cut portion including a part of the gate.
The heat insulation means can be in the form of a gap defined between the mold and the nozzle and filled with air, ceramic or the like.
It is desirable to position the gate cut portion as closer as possible to the cavity. However, the temperature of the metal in the gate cut potion decreases drastically by bringing the gate cut portion to the cavity and the movable mold whose temperature is low. Accordingly as the gate cut portion comes closer to the cavity, it comes closer to a low-temperature product in the cavity or to the low-temperature movable mold plate, resulting in a rapid drop of the temperature of the metal in the gate cut portion. It is therefore desirable to select a position as closer as possible to the cavity and a position allowing the temperature of metal in the gate cut portion to be kept at an appropriate level after the gate cutting.
In case of heating of the nozzle by the heating means, the further away from the heating means it goes, the lower the temperature of the metal becomes, whereas the closer to the nozzle tip it comes, the larger the rate of drop of the metal temperature becomes. Thus, in the nozzle of the present invention, the position of the gate cut portion is determined in accordance with the gate cut portion position determination manner which will be described later. In case of the nozzle having the thus determined gate cut portion, it is preferred to keep the metal temperature at any temperature in the range of 400xc2x0 C. to 580xc2x0 C. when the metal is magnesium alloy for example.
If the temperature of metal in the gate cut portion is higher than the upper limit of this range, the metal in the nozzle runner heated by the heating means may reach a temperature exceeding the melting point, with the result that the molten metal may possibly leak out of the gate cut portion. On the contrary, if the temperature is lower than the lower limit of this range, it will take more time to melt the metal solidified in the vicinity of the gate cut portion, resulting in an elongated cycle time of the injection molding, which will make it unsuitable for the practical use.
When the molten metal is a magnesium alloy, the present inventors have determined the optimum position and hold temperature of the gate cut portion in accordance with the manner which will be described later.
As a result, it has been proved that the gate cut portion should be positioned in substantially the middle region between the nozzle tip and the leading end portion of the induction heating coil. After repeated trial and error, it has been proved that the solidified state of metal in the gate cut portion can stably be held at the temperature near the melting point while keeping the metal in the nozzle runner in its molten state upon the mold opening, by providing a control of the heating temperature so as to allow the temperature of the magnesium alloy in the gate cut portion to exist in the range of 520xc2x0 C. to 560xc2x0 C.
It is thus possible for the magnesium alloy to be injection molded at an optimum cycle time, thereby eliminating any risk of leakage of the molten metal out of the gate cut portion upon the mold opening.
In place of the heat insulation means or in conjunction with the formation of the heat insulation means, the nozzle body may be made of ceramic, the periphery of the nozzle being covered with a metallic outer tube, the metallic outer tube having an induction heating coil wound therearound such that molten metal is flown into a gap defined between the outer tube metal and the ceramic nozzle body.
According to this construction, the nozzle body is formed of ceramic having a low thermal conductivity, so that it is possible to reduce the quantity of heat conducted from the molten metal in the gate to the mold and to thereby suppress the drop of temperature of metal in the gate. A further effectiveness is achieved by the formation of the heat insulation means around the nozzle.
In this case, a difference exists in the thermal expansion coefficient between the metal forming the fixed mold plate and ceramic forming the nozzle, so that there may be formed a gap between the fixed mold plate and the nozzle upon the injection of molten metal into the cavity, which may possibly result in a backflow of the molten metal in the cavity into the gap.
This is the reason why the gap is formed between the nozzle and the fixed mold plate so that molten metal fills up this gap. Formation of a hole leading to the gap in the nozzle enables the filling of the molten metal to be effected simultaneously with the injection of metal. The molten metal filled into the gap has the effect of not only preventing a backflow thereof from the cavity as a result of blockage of the gap, but also effectively conducting the heat from the heating means through the metallic outer tube to the nozzle.
Heat radiation means may be disposed on the nozzle at the tip thereof so as to accelerate heat radiation from the metal upon the mold opening. The heat radiation means may be in the form of a member having a high heat radiating property affixed to the tip of the nozzle or in the form of a cooling air flow passage formed at the tip of the nozzle.
Provision of such heat radiation means can accelerate a rapid solidification of metal in the nozzle tip portion upon the mold opening. On the other hand, the periphery of the gate cut portion is heat insulated by the heat insulation means so that the metal in the gate cut portion is kept at a certain temperature or above. This enables the position of the gate cut portion to come as closer as possible to the nozzle tip.
In the mold, the position of the gate cut portion is determined in accordance with the gate cut portion position determination manner which follows.
The manner comprises the steps of disposing heating means for heating metal existing in the nozzle, at any position on the nozzle; disposing a plurality of temperature measurement points for measuring the temperature of the metal existing in the nozzle, at a predetermined interval, in a region from the tip of the nozzle to the heating means; selecting at least one temperature control target point as the reference for the temperature control, out of the plurality of temperature measurement points; providing a control of the heating means upon the mold opening such that metal in at least a portion provided with the heating means is put in molten state and that the temperature of the temperature control target point is kept at a constant level which is lower than the melting point of the metal; measuring the distribution of temperatures of the other ones of the plurality of measurement points when the temperature of the temperature control target point is kept constant; determining, from the results of the measurement, an optimum temperature region where the solidified state of the metal is stably maintained upon the mold opening and where the temperature of the metal solidified is closest to the melting point of the metal; and setting a gate cut portion within the optimum temperature region.
It is preferred upon the creation of a temperature distribution graph of the plurality of temperature measurement points based on the results of the measurement that conditions are appropriately selected including the positions of the nozzle, nozzle heat radiation means, nozzle heat insulation means or heating means so as to ensure that the temperature distribution graph has at least one portion where the gradient of the graph becomes gentle or substantially flat so that the substantially flat portion is defined as the optimum temperature region.
The temperature gradient can be controlled by providing various heat insulation means or by providing heat radiation means.
When the molten metal is a magnesium alloy, the optimum temperature region is preferably controlled so as to lie within the range of 520xc2x0 C. and 560xc2x0 C. by use of the heat insulation means or the heat radiation means.
According to another example of the mold for the hot-runner injection molding machine, there is provided a mold which includes a movable mold plate having a cavity and which includes a fixed mold plate having a nozzle for injecting molten metal into the cavity and having heating means for heating metal existing in the nozzle, the mold comprising a pin or ejector pin disposed on the movable mold plate, the ejector pin capable of traversing the cavity to project up to the gate cut portion; a driver arranged to advance and retreat the ejector pin between the protruded state and the retracted state; and drive control means for providing a control of drive of the driver.
This construction enables the gate cut portion to be compulsorily opened by the ejector pin previous to the metal injection.
The drive control means outputs a command allowing the ejector pin to project when the temperature of metal in the gate cut portion reaches a predetermined temperature after the mold closing.
Thus, by allowing the ejector pin to project to compulsorily open the gate cut portion when the temperature of the metal solidified in the gate cut portion has reached a preset temperature, e.g., 500xc2x0 C. in case of magnesium alloy having the melting point of 596xc2x0 C., after the mold closing, it is possible to shorten the cycle time of the hot-runner injection molding and to easily manage the temperature of the gate cut portion.